Ellipsometry is a nondestructive analysis technique for studying physical properties of samples based on changes in the polarization state of light caused by the interaction of a light beam with the sample. In all ellipsometer systems, a light beam having a known state of polarization is reflected off or transmitted through a sample. The difference between polarization states of the light beam before and after interaction with the sample can be used to calculate characteristics of the sample, such as film thickness, index of refraction, and absorption coefficients.
One common use for ellipsometry is the analysis of thin-film layers, such as a dielectric layer, on semiconductor samples. Reflection ellipsometry is presently considered to be the best method for measuring thicknesses of thin layers on semiconductors. While a number of parameters can be used to define a semiconductor sample with a coated dielectric layer, at least five parameters should be measured to completely characterize the sample: indexes of refraction of the film and substrate, absorption coefficients of the film and substrate, and the thickness of the film. For a given wavelength, however, ellipsometers allow measurement of only two parameters, .psi. and .delta., which relate to the magnitude of and phase shift between the S- and P-orthogonal polarization states of a beam reflected from a sample, respectively. If R.sub.P and R.sub.S are complex amplitude reflectivities of the polarized light reflected from the sample, then: EQU R.sub.P /R.sub.S =tan .PSI. exp (i.delta.)
Because only .psi. and .delta. can be measured, there is a so-called "order ambiguity," a classic problem with ellipsometry.
Another common problem with ellipsometry is that the measurement data is affected by intensity variation and background noise that ultimately set the limit on the accuracy and resolution of the instruments. Another problem, which prevents an ellipsometer from being used as a compact in-line instrument, is its inherent low-speed data collection rate. This data collection rate is particularly affected by time consuming alignment procedures and electronically controlled and mechanically rotating polarizer elements. The low speed of rotation of such polarizing elements sets an ultimate limit on the speed of data collection.
Electromechanical positioning problems due to vibration and difficulties in controlling and aligning rotating parts introduce further error into the test results. For example, the mechanically rotating polarizing elements have an inherent inaccuracy in its angular setting of about 0.05.degree.. The phase difference between the rotating polarizing elements and the rotary encoder is also difficult to control.
Instead of rotating elements, some ellipsometers use a phase-modulation technique which utilizes electronically-controlled phase retardation. The signal-to-noise ratio is improved by eliminating the electromechanical elements. However, these ellipsometers cannot compensate for low-frequency drift which occurs in the phase modulation loop due to variations in the temperature and ambient conditions.
Photoelectric type modulation (PEM) ellipsometers, a type of phase modulation ellipsometer, present additional problems because they are resonance devices with a maximum excitation frequency between 70 and 100 kHz. The limited modulation range limits the capability of available demodulation techniques and also sets a limit on the speed of data collection.